What are these knobs for, anyway?

I have a recent-vintage (maybe 10 or so years old) Fender P-bass, 4 strings, 2 pickups, 2 stacked knobs.I, the lead guitarist in our band, and my bass teacher have tried to figure out what the knobs do.They are stacked. The lower front (toward the neck) knob seems to be the switch between pickups, and the upper front appears to be volume.That leaves the upper and lower knobs in the back. Any ideas? Thanks.